arXiv subjects

Ivica Picek

Publications and source records attributed to Ivica Picek.

18 recordsLinked to original sources

arXiv2019

BSM Matter providing Neutrino Masses and Gauge Unification

I present several scenarios developed in Zagreb, in which TeV-scale particles belonging to non-trivial weak-isospin multiplets give rise to neutrino-mass mechanisms different from conventional type I, II and III seesaw models. Two dim 9 tree-level mechanisms, presented first, provide an appealing testability of their exotic TeV-scale particles at the LHC. These models are not genuine, since their particles also provide competing dim 5 loop contributions. The loop-models presented next are genuine, without competing tree-level contributions. Among them, the three-loop model involves high-order weak multiplets leading to Landau poles. The one-loop model with scalar triplet as the largest multiplet, in addition to good UV properties, provides the particle set promising for gauge coupling unification. Therefore, it served us as a starting point for a study of SU(5) embedding of additional particles leading to viable unification scenarios. To distinguish among them begs for additional principle which reigns over particle completion and eventual dark matter considerations.

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arXiv2017

Renormalizable SU(5) Completions of a Zee-type Neutrino Mass Model

We explore the potential of a selected model of radiative neutrino masses to be implemented in a renormalizable SU(5) unification framework. The Zee-type model under consideration uncovers the SU(5) representations in which the new fields are embedded and which may contain also other light states leading to the unification of gauge couplings. We perform an exhaustive search which reveals specific patterns of new states and demonstrate that such patterns are consistent with a general choice of relevant scalar potential. It turns out that all of the specific scenarios which lead to successful unification include the colored scalars testable at the LHC.

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arXiv2017

Extended Higgs Sectors in Radiative Neutrino Models

Testable Higgs partners may be sought within the extensions of the SM Higgs sector aimed at generating neutrino masses at the loop level. We study a viability of extended Higgs sectors for two selected models of radiative neutrino masses: a one-loop mass model, providing the Higgs partner within a real triplet scalar representation, and a three-loop mass model, providing it within its two-Higgs-doublet sector. The Higgs sector in the one-loop model may remain stable and perturbative up to the Planck scale, whereas the three-loop model calls for a UV completion around 106 GeV. Additional vector-like lepton and exotic scalar fields, which are required to close one- and three-loop neutrino-mass diagrams, play a decisive role for the testability of the respective models. We constrain the parameter space of these models using LHC bounds on diboson resonances.

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arXiv2017

Radiative neutrino models in light of diphoton signals

Viable explanations of a hinted 750 GeV scalar resonance may be sought within the extensions of the SM Higgs sector aimed at generating neutrino masses at the loop level. We confront a compatibility with the 750 GeV diphoton excess for two recent models which do not need to impose ad hoc symmetry to forbid the tree-level masses: a one-loop mass model providing the H(750) candidate within its real triplet scalar representation and a three-loop mass model providing it within its two Higgs doublets. Besides accounting for the 750 GeV resonance, we demonstrate that these complementary neutrino-mass scenarios have different testable predictions for the LHC which should show up soon as more data is accumulated during the ongoing 13 TeV run.

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arXiv2016

A Critical Analysis of One-Loop Neutrino Mass Models with Minimal Dark Matter

A recent paper investigated minimal R$ν$MDM models with the type T1-iii and T3 one-loop topologies. However, the candidate most-minimal model does not possess an accidental symmetry - the scalar potential contains an explicit symmetry breaking term, rendering the dark matter unstable. We present two models that cure this problem. However, we further show that all of the proposed minimal one-loop R$ν$MDM models suffer from a second problem - an additional source of explicit $Z_2$ symmetry breaking in the Yukawa sector. We perform a more-general analysis to show that neutrino mass models using either the type T3 or type T1-iii one-loop topologies do not give viable minimal dark matter candidates. Consequently, one-loop models of neutrino mass with minimal dark matter do not appear possible. Thus, presently there remains a single known (three-loop) model of neutrino mass that gives stable dark matter without invoking any new symmetries.

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arXiv2015

Scotogenic R\nuMDM at Three-Loop Level

We propose a model in which the radiative neutrino (Rν) masses are induced by fermion quintuplet and scalar septuplet fields from the minimal-dark-matter (MDM) setup. In conjunction with the 2HDM fields, on top of which our model is built, these hypercharge zero fields and additional scalar quintuplet lead to an accidental DM-protecting Z_2 symmetry and establish the R\nuMDM model at the three-loop level. We assess the potential for discovery of quintuplet fermions on present and future pp colliders.

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arXiv2013

Radiative Neutrino Mass with Scotogenic Scalar Triplet

We present radiative one-loop neutrino mass model with hypercharge zero scalar triplet in conjunction with another charged singlet scalar and an additional vectorlike lepton doublet. We study three variants of this mass model: the first one without additional beyond-SM symmetry, the second with imposed DM-stabilizing discrete Z_2 symmetry, and the third in which this Z_2 symmetry is promoted to the gauge symmetry U(1)_D. The two latter cases are scotogenic, with a neutral component of the scalar triplet as a dark matter candidate. In first scotogenic model the Z_2-odd dark matter candidate is at the multi-TeV mass scale, so that all new degrees of freedom are beyond the direct reach of the LHC. In second scotogenic setup, with broken U(1)_D symmetry the model may have LHC signatures or be relevant to astrophysical observations, depending on the scale of U(1)_D breaking.

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arXiv2013

New Scotogenic Model of Neutrino Mass with $U(1)_D$ Gauge Interaction

We propose a new realization of the one-loop radiative model of neutrino mass generated by dark matter (scotogenic), where the particles in the loop have an additional $U(1)_D$ gauge symmetry, which may be exact or broken to $Z_2$. This model is relevant to a number of astrophysical observations, including AMS-02 and the dark matter distribution in dwarf galactic halos.

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arXiv2013

Neutrino Masses and TeV-scale Particles Testable at the LHC

We consider a scenario in which TeV-scale particles belonging to weak-isospin multiplets higher than triplets lead to novel seesaw mechanisms different from conventional type I, II and III seesaw models. Besides an appealing testability of these mechanisms at the LHC, the model with Majorana quintuplets with imposed discrete symmetry may provide viable dark matter candidate.

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arXiv2013

Enhancement of $h \to γγ$ by seesaw-motivated exotic scalars

We examine the role of seesaw motivated exotic scalars in loop-mediated Higgs decays. We consider a simple TeV-scale seesaw model built upon the fermionic quintuplet mediator in conjunction with the scalar quadruplet, where we examine portions of the model parameter space for which the contributions of charged components of the scalar quadruplet significantly increase the $h \to γγ$ decay rate. The most significant change in the diphoton width comes from a doubly charged scalar Φ^{--} which should be the lightest component in the scalar quadruplet. There is a mild suppression of the $h \to Z γ$ decay width by a factor 0.9 -- 0.7 in the part of the parameter space where the $h \to γγ$ decay width is enhanced by a factor 1.25 -- 2.

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arXiv2012

Critique of Fermionic R\nuMDM and its Scalar Variants

We examine the stability of minimal dark matter (MDM) particle-candidates in the setup in which they participate in radiative neutrino (Rν) masses. We first point out the existence of an additional renormalizable term in recently proposed R\nuMDM Lagrangian, which violates the claimed accidental Z_2 symmetry and spoils the stability of the fermionic MDM quintuplet component. We then explore the viability of R$ν$MDM variants based on scalar MDM multiplets. There are ubiquitous super-renormalizable terms in the scalar potential which make these scalar multiplets unstable.

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arXiv2012

TeV-scale Seesaw with Quintuplet Fermions

We propose a new seesaw model based on fermionic hypercharge zero weak quintuplet in conjunction with additional scalar quadruplet which attains an induced vev. The model provides both tree-level seesaw ~ v^6/M^5 and a loop-suppressed radiative ~ (1 / 16 π^2) v^2/M contributions to active neutrino masses. The empirical masses m_ν~ 10^{-1} eV can be achieved with M ~ TeV new states, accessible at the LHC. For 5 fb^{-1} of accumulated integrated luminosity at the LHC, there could be ~ 500 doubly-charged Σ^{++} or \bar{Σ^{++}} fermions with mass M_Σ= 400 GeV, leading to interesting multi-lepton signatures. The neutral component of the fermion quintuplet, previously identified as minimal dark matter candidate, becomes unstable in the proposed seesaw setup. The stability can be restored by introducing a Z_2 symmetry, in which case neutrinos get mass only from radiative contributions.

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arXiv2011

Exotic Seesaw-Motivated Heavy Leptons at the LHC

We study the LHC potential for discovering TeV-scale SU(2)_L 5-plet fermions introduced recently to explain small neutrino masses. We show that the Drell-Yan production and the decays of new exotic Sigma leptons are testable at the LHC. Their production is abundant due to nontrivial electroweak gauge charges. For 1 fb^-1 of integrated luminosity at the present LHC sqrt{s}=7 TeV, there can be 270 Sigma-Sigmabar pairs produced for M_Sigma = 400 GeV. Besides producing same-sign dilepton events, they could lead, due to a chosen small mixing between heavy and light leptons, to ~10 golden decays Sigma^{+++}(Sigma^{+++}-bar) --> W^\pm W^\pm l^\pm with a specific decay signature.

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arXiv2011

Testing New TeV-scale Seesaw Mediators at the LHC

We propose TeV-scale Dirac fermions producing Majorana masses of the known neutrinos via tree-level seesaw, different from standard type I and III seesaw. The employed weak five-plet with nonzero hypercharge leads to new seesaw formula m_ν~ v^6/M^5 and to empirical masses m_ν~ 10^{-1} eV for M ~ TeV new states. For a limited range of the parameter space, where M < a few 100 GeV, the proposed mechanism is testable at the LHC via characteristic decays of Dirac type heavy leptons, produced by a Drell-Yan fusion.

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arXiv2010

Role of Higher Fermion Representations in TeV-scale Seesaw

We consider a scenario in which additional vectorlike TeV-scale fermions belonging to higher weak-isospin multiplets provide new seesaw mediators. If these fermions have non-zero hypercharge, their tree-level exchange produces novel seesaw mechanism different from type I and III seesaw. In order to produce Majorana masses for light neutrinos, new Dirac seesaw mediators are constrained by the SM gauge symmetry to belong to a weak triplet and a five-plet. The latter, in conjunction with two isospin 3/2 scalar multiplets, leads to new seesaw formula m_ν~ v^6/M^5. It reproduces the empirical masses m_ν~ 10^{-1} eV by M < TeV new states, testable at the LHC.

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arXiv2010

Novel TeV-scale seesaw mechanism with Dirac mediators

We propose novel tree level seesaw mechanism with TeV-scale vectorlike Dirac mediators that produce Majorana masses of the known neutrinos. The gauge quantum number assignment to the Dirac mediators allows them to belong to a weak triplet and a five-plet of nonzero hypercharge. The latter leads to new seesaw formula m_ν~ v^6/M^5, so that the empirical masses m_ν~ 10^{-1} eV can be achieved by M ~ TeV new states. There is a limited range of the parameter space with M < a few 100 GeV where the tree level contribution dominates over the respective loop contributions and the proposed mechanism is testable at the LHC. We discuss specific signatures for Dirac type heavy leptons produced by Drell-Yan fusion at the LHC.

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arXiv2008

New Dipole Penguin Contribution to K --> pi pi decays

We point out that the standard chromomagnetic penguin dipole operator has a counterpart corresponding to off-shell momenta for external quarks. By employing the chiral quark model, we show that this new dipole penguin operator has the same bosonisation as the standard Q_6 operator. Accordingly, this new operator enlarges by ~ 5 % the referent Q_6 contribution, which gives the dominant contribution to the CP-violating ratio epsilon'/epsilon and also gives an important contribution to the Delta I = 1/2 amplitude.

hep-ph↗

arXiv2008

Nondecoupling of a terascale isosinglet quark and rare K and B decays

We examine recent extensions of the standard model with an up-type vectorlike isosinglet T quark that mixes dominantly with the top quark. We take under scrutiny the nondecoupling effects which may reveal such a new heavy fermion through loop diagrams relevant for rare decays such as K-->pi nu nu-bar, B-->pi(K) nu nu-bar, and B_{s,d}-->mu^+ mu^-. After demonstrating in detail the cancellation between the leading nondecoupling terms, we show that two residual forms ~ s^2 ln{m_T^2} and ~ s^4 m_T^2 act in complementary way, so that the maximal allowed values of the decay rates are practically independent of m_T. While they correspond to ~ 20% or ~ 30% corrections to the SM rates for K-->pi nu nu-bar and B-->pi(K) nu nu-bar, an increase by ~ 50% for B_{s,d}-->mu^+ mu^- decays offers a possibility to reveal an additional isosinglet state by measurements of these decays at the Large Hadron Collider.

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